The source mechanism of the Portugal earthquake of February, 1969, is studied primarily on the basis of long-period surface wave data. The Azores-Gibraltar seismic belt crosses the Eastern North Atlantic Ocean and defines a compressive plate boundary in the vicinity of the epicentre. The major objective of this study is to understand the nature of the boundary which is consuming the two oceanic plates, neither of which sinks into the asthenosphere. The fault plane solution is a thrust with a small strike slip component. The geometry of the planar distribution of the aftershocks distinguishes the fault plane (dip direction = N35°W, dip angle = 52°) from the auxiliary plane (dip direction = S9°E, dip angle = 41°). The seismic moment is 6.0 × 10 27 dyne-cm. Other source parameters are also estimated. The fault plane extends through the upper half of the oceanic lithosphere with dip 52°. The relationship between the source mechanism of the earthquake and certain atypical seamounts near the epicentre is discussed. Repeated thrusting on a plane cutting through the lithosphere causes the bending of the lithosphere. The bending can lead to the formation of a second fault plane on which subsequent slip will take place. Regional uplift or subsidence of oceanic crust must then occur. Such a process of the shortening of oceanic crust is believed to produce massive seamounts west of the Straits of Gibraltar.